A high peak-to-average-power-ratio of a transmit signal is one of the greatest problems of multi-carrier systems, for example, orthogonal frequency division multiplexing (OFDM) or discrete multi-tone (DMT) systems. High peak values drive non-linear power amplifiers during operation in non-linear regions of their amplifying characteristics. Amplifiers being operated in non-linear regions of their characteristics cause inter-modulation products, modulations among sub carriers and out of band radiation. To prevent these effects a power amplifier must be operated in the linear region of its characteristics introducing large input back-offs for the case of higher peaks being present. The problem of higher peak-to-average-power ratios can, in some applications, even compensate or annul the benefits of multi-carrier modulations. Hence, the reduction of the peak-to-average-power-ratio is an essential topic in multi-carrier systems.
A number of techniques has been proposed to deal with the problem of high peak-to-average-power ratios in orthogonal frequency division multiplexing. The following list provides a summary of the state of the art concepts:                G. R. Hill, M. Faulkner, and J. Singh, “Reducing the Peak-to-Average Power Ratio in OFDM by Cyclically Shifting Partial Transmit Sequences,” Elect. Lett., vol. 36, no. 6, March 2000, pp. 560-561.        A. E. Jones, T. A. Wilkinson, and S. K. Barton, “Block Coding Scheme for Reduction of Peak to Mean Envelope Power Ratio of Multicarrier Transmission Scheme,” Elect. Lett., vol. 30, no. 22, December 1994, pp. 2098-2099.        B. S. Krongold and D. L. Jones, “PAR Reduction in OFDM via Active Constellation Extension,” IEEE Trans. Broadcast., vol. 49, no. 3, September 2003, pp. 258-268.        X. Li and L. J. Cimini, Jr., “Effect of Clipping and Filtering on the Performance of OFDM,” IEEE Commun. Lett., vol. 2, no. 5, May 1998, pp. 131-133.        S. H. Müller and J. B. Huber, “OFDM with Reduced Peak-to-Average Power Ratio by Optimum Combination of Partial Transmit Sequences,” Elect. Lett., vol. 33, no. 5, February 1997, pp. 368-369.        S. H. Müller and J. B. Huber, “A Comparison of Peak Power Reduction Schemes for OFDM,” Proc. IEEE GLOBECOM '97, Phoenix, Ariz., November 1997, pp. 1-5.        R. O'Neill and L. B. Lopes, “Envelope Variations and Spectral Splatter in Clipped Multicarrier Signals,” Proc. IEEE PIMRC '95, Toronto, Canada, September 1995, pp. 71-75.        J. Tellado, Peak-to-average-power Reduction for Multi-carrier Modulation, Ph.D. dissertation, Stanford Univ., 2000.        
A comprehensive overview of the above-mentioned techniques including some further references can be found in, S. Hee Han and J. Hong Lee, “An Overview of Peak-to-Average Power Ratio Reduction Techniques for Multi-carrier Transmission”, IEEE Wireless Communications, April 2005, pp. 56-65.
One of the most promising solutions is the tone reservation method. In this method, several sub-carriers are reserved for transmission of peak-to-average-power-ratio reduction tones. The peak-to-average-power-ratio reduction tones are determined so as to minimize the peak-to-average-power-ratio of the original transmission signal. Although the corresponding peak-to-average-power-ratio reduction results are promising, this method suffers from loss in spectral efficiency due to the reservation of exclusive tones for the purposes of peak-to-average-power-ratio reduction. In addition, power efficiency is reduced as part of the transmit power is invested not into data sub-carriers, but on peak-to-average-power-ratio reduction sub-carriers. The tone reservation method is currently under consideration by the third generation partnership project (3GPP) within their technical specification group radio access network as one of the physical layer aspects for evolved Universal Terrestrial Radio Access (UTRA), cf. 3rd Generation Partnership Project (3GPP), “Physical layer aspect for evolved Universal Terrestrial Radio Access (UTRA)”, June 2006.